Head trauma. Repeated blows to the head — think boxers, like Muhammad Ali — likely increases one’s risk of developing Parkinson’s, but “at this point, we don’t know with 100 percent certainty that it causes the disease,” says Liana Rosenthal, M.D., assistant professor of neurology at the Johns Hopkins University School of Medicine. “Not everyone who gets head trauma is going to develop Parkinson’s, and we can’t say for sure that there’s a type, severity or frequency of head trauma that increases your risk.”